U.S. State Department spokesman P.J. Crowley, speaking to reporters in Washington, declined to comment on the situation.

"We are aware of the Iranian claims," he said. "I have no information."

Tehran originally refused to acknowledge Amiri's involvement in Iran's disputed nuclear program, which the United States suspects is being used to develop nuclear weapons. Iran says it is aimed at generating electricity.

Mehmanparast said Amiri was being detained in the United States.

"He is among 11 jailed Iranians in America," he said, without elaborating.

Iran has said the United States was involved in Amiri's disappearance, which Washington has denied.

Amiri disappeared more than three months before Iran disclosed the existence of its second uranium enrichment site, near the central holy Shi'ite city of Qom.

The underground plant was kept secret until September.

In 2007, Iran's police chief suggested that a former deputy defence minister, Ali Reza Asgari who disappeared in Turkey that year, had been kidnapped by Western intelligence. Israel and the United States deny any involvement in his disappearance.

At the time, Turkish newspapers reported that Asgari had information about Iran's nuclear program. Turkish, Arabic, and Israeli media have said he defected to the West, but his family dismissed the idea.

Western suspicions about Iran's atomic agenda were strengthened when it said last month it would build 10 more uranium enrichment sites like its main one at Natanz, which is monitored by the International Atomic Energy Agency.

Iran says it needs 20 uranium enrichment sites to generate fuel for its nuclear power plants. Iran has one nuclear power plant, being built by Russia.

Uranium enrichment can be calibrated to yield fuel either for nuclear power plants or the fissile core of a nuclear bomb.